1. Field of Invention
The present invention relates to the field of electronic components, and in particular to a programmable analog tapped delay line filter.
2. Description of Related Art
Adaptive equalizers in communication systems have been used in the transmitter and receiver to compensate channel Inter Symbol Interference (ISI).
In order to compensate for linear ISI in a receiver, an equalizer in the form of a tapped delay line filter may be installed between the receiver preamplifier and threshold detector to shape the pulse at the input of the decision device.
FIG. 1 illustrates a conceptual schematic of a conventional tapped delay line analog filter 100. The filter 100 comprises: an input signal denoted 1; one or more delay cells denoted 3, 5, and 7; four taps 9, 11, 13, and 15, each with a tap coefficient denoted C0, C1, C2, and C3 respectively; and an output signal 17.
As illustrated in FIG. 1, input signal 1 passes through delay cells 3, 5, and 7, whose function is to produce delayed versions of the input signal 1. Moreover, output signal 17 is a weighted sum of the delayed inputs, expressed as:
xe2x80x83y(t)=C0*x(t)+C1*x(txe2x88x92T)+C2*x(txe2x88x922T)+C3*x(txe2x88x923T)+
where T represents a constant delay for each delay cell.
In order for the equalizer to be optimally adaptive, the tap coefficients should be variable both in value and in sign, wherein each tap coefficient varies independently of any other tap coefficient. Furthermore, single ended delay cells may be sensitive to factors such as power supply noise, common-mode coupling, and substrate noise.
Although conventionally accepted analog tapped delay line filters operating at microwave frequencies (i.e. several gigahertz and higher) are able to alleviate the problem of equalizing communication channels, these filters have either fixed tap coefficients (e.g. fixed sign, fixed value, etc.) or employ single ended delay cells sensitive to noise errors.
The present invention provides the method and apparatus for a programmable analog tapped delay line filter.
In one embodiment of the present invention, a programmable analog tapped delay line filter comprises an input line, an output line, and one or more gaincells or taps. The input and output lines each comprises a cascade of one or more differential delay cells. The one or more gaincells are coupled between the input and output lines. Furthermore, the input and output lines are terminated in impedances and the filter produces one or more outputs.
Moreover, the one or more differential delay cells may be implemented with either transmission lines or L-C sections. The one or more gaincells or taps may be differential amplifiers with programmable gain, whose output current is proportional to the input voltage; and the gaincells or taps may be implemented with active devices such as bipolar junction transistors, Metal-Oxide-Semiconductor FETs, Heterojunction devices, etc.
The programmable analog tapped delay filter receives differential values of an input signal at two distinct inputs; delays the difference of the inputs by each of the one or more differential delay cells; and outputs one or more output signals.